1. Field of the Invention
The present invention relates to a color image forming apparatus for forming a color image by mixing a plurality of coloring agents.
2. Related Background Art
Hitherto, a conventional electrophotographic color image forming apparatus formed a color image by a method comprising the steps of: a latent image forming process in which a color image, the color of which has been decomposed into four colors, that is, yellow, magenta, cyan and black, is exposed and formed on the surface of an image carrier such as a photosensitive drum, which has been uniformly charged, by a laser beam serving as an exposure means according to the present invention; a development process for developing the latent image formed on the photosensitive drum into a toner image (a visible image); a transfer process for transferring the toner image formed on the photosensitive drum to the surface of recording paper; and a fixing process for fixing the toner image formed on the recording paper.
For example, an electrophotographic color copying machine serving as an application example of the color image forming apparatus according to the present invention reads an original color image while decomposing the color into three colors, that is, red (R), green (G) and blue (B) and they are color-converted into CMY density signals, which are the complementary signals of RGB so that a color signal of the subtractive mixture type which uses toner is obtained.
Usually, a black (BK) signal is formed in addition to the YMC density signals so that the image is formed by using color toners of four colors, that is, yellow, magenta, cyan and black. That is, the black toner is used to perform the under color removal in which black realized by toner is employed in place of black obtained from the YMC three-color mixture. When the under color removal is performed by using the black toner, it is considered that the following effects can be obtained:
(1) The density reproducibility in the high density image portion can be improved.
(2) The color reproducibility in a colorless image region can be stabilized.
(3) The sharpness of the image can be improved.
(4) The running cost can be reduced because the quantity of the consumption of the toner can be reduced.
In order to further improve the above-described effects (1), (2) and (3), a method has been ordinarily employed in which the black toner is made to form the uppermost layer of the formed image.
On the other hand, in an office, a color copying machine has been used not only to copy a color original document but also used to copy a white and black image. Therefore, the color copying machine must be able to function as the conventional white and black copying machine and as well as have excellent cost performance. Therefore, the toner for the conventional white and black copying machine is sometimes used because of its advantage of the low cost in comparison to that of the color toner.
However, there arises a problem in that, when the black toner for the white and black copying machine is used in the color copying machine in such a manner that it is placed at the uppermost layer of the formed image, the glossiness of the surface of the color region and that of the colorless region become different from each other on the same image and thereby a person who observes the image feels strange.
For example, when a full color human figure or an image of a fruit as shown in FIG. 22 is color-reproduced in such a manner that the skin (Y+M), the clothes (Y), an apple (M+Y) and a muscat (C+Y) are respectively reproduced by using the color toners, the surface of each of the images formed by the above-described color toners displays a high glossiness. However, the images of the hair, the pupil of the eye and the eyebrow formed by the black toner display insufficient glossiness. As a result, the glossiness of each of the color regions and that of each of the colorless regions become different from one another at the surface thereof. In particular, since a severe evaluation is usually made about the image of a human figure, the quality of the image becomes unsatisfactory for a user.
The reason why the above-described uneven glossiness takes place will now be described briefly. It is preferable that the toner has a low melting point (140.degree. to 150.degree. C.) so as to be dissolved and mixed with other color toners in order to improve the color reproducibility of a color image. On the other hand, a black toner for use in a conventional white and black copying machine is arranged to possess a high melting point (180.degree. to 190.degree. C.) for the purpose of raising the copying speed and improving the durability because of its high frequency in use.
Therefore, although the color toners can be dissolved by heat in the fixing process in which the toners are fixed to a recording paper sheet, the black toner cannot be satisfactorily dissolved. As a result, the colorless black toner image regions involve uneven portions on their surfaces in a microscopic view. Therefore, light made incident upon the paper surface can be diffused and reflected, causing the glossiness of the surface of the image to be deteriorated. On the other hand, the surface of the image region formed by the color toners is smooth enough to regularly reflect light made incident upon the surface of the paper. As a result, the above-described region displays excellent glossiness.
Hitherto, there has been suggested a method of changing the glossiness of a formed image in such a manner that the temperature of the fixing roller is changed. Another method has been disclosed in which the speed of the fixing roller is changed.
Each of the above-described two methods is arranged in such a manner that the heating value to be given to the toner is changed to adjust the degree at which the toner is melted so as to change the surface condition of the image. Thus, the glossiness is changed.
However, the above-described methods are necessarily arranged in such a manner that the essential factors for forming the image, such as the color mixture characteristics and fixing characteristics, are changed undesirably. Therefore, an image exhibiting excellent quality cannot be obtained because the glossiness, the color tone and adhesive force can be changed. Furthermore, since the fixing process is performed in a state where the fixing conditions are not suitable for the life of the fixing roller, the fixing roller can easily be deteriorated.
The conventional full color copying machine has been arranged in such a manner that the melting points of the magenta, cyan, yellow and black toners are made to be closer to one another as much as possible to prevent the undesirable change in the glossiness of one color.
However, the melting point of the black toner for use in the above-described conventional color copying machine is lower than that of the toner for use in the ordinary monochromatic copying machine. Therefore, the glossiness of the black image, for example, that of a black line image such as a black character, is raised excessively. Therefore, there arises a problem in that the formed image cannot easily be recognized depending upon the way of placing the light source for use to observe the formed image.
Furthermore, in a full color copying machine or the like, the four color decomposition algorithm for obtaining chromaticity (color tone) required by a user has different optimum value which is determined depending upon the type and the characteristics of the recording medium such as the surface flatness, the chromaticity and the whiteness of the recording medium and as well as a fact that the recording medium is used to form a transparent image or a reflected image. Therefore, images exhibiting high quality cannot always be obtained regardless of the type of the recording medium. Furthermore, in order to obtain an image (hereinafter called an "OHP image") for use in an over head projector, the image must have high transparency. Accordingly, there arises a problem in that, if the transparency of each of the toner images is different depending upon the hue, any means capable of overcoming this fact must be provided in order to obtain an image exhibiting high quality.